1. Field of the Invention
The present invention relates to a feedback type air fuel ratio controlling system in which exhaust gas composition concentrations in an engine are detected to feedback control supply amount of fuel or air to the engine to thereby maintain the air fuel ratio at a set value.
2. Description of the Prior Art
A feedback type air fuel ratio controlling system for controlling an air fuel ratio of a mixture of an internal combustion engine or an air fuel ratio of an exhaust gas flowing into an exhaust gas purifier disposed in the exhaust system of the engine, that is, a ratio between total amount of air and total amount of fuel which are supplied to a passage extending from the inlet system of the engine to the exhaust system of the engine just upstream side of the exhaust gas purifier (hereinafter referred to as a total air fuel ratio) in accordance with a detected actual total air fuel ratio has conventionally been known wherein the actual air fuel ratio of the engine detected from the exhaust gas composition concentrations is used to feedback control supply of fuel to the engine, supply of auxiliary air to the engine inlet system or supply of secondary air to the engine exhaust system for maintaining the actual total air fuel ratio of the engine within an extremely narrow range around a set value, whereby the exhaust gas may be purified. Especially, with an exhaust gas purifier in the form of a ternary or three-way catalytic converter, it is possible to simultaneously reduce amounts of three harmful compositions such as HC, CO and NOx contained in the exhaust gas by determining the aforementioned set value to the stoichiometric air fuel ratio. Such prior art feedback type air fuel ratio controlling systems will be described herein with reference to FIGS. 1 and 2. FIG. 1 shows a well known feedback type air fuel ratio controlling system, as disclosed in U.S. Pat. Nos. 3,903,853; 3,745,768; 4,020,813 and 3,960,118, for example, which controls the air fuel ratio of a mixture in the engine inlet system, that is, feedback controls supply amount of fuel or auxiliary air to the engine inlet system by using the detecting output of an exhaust gas sensor. The controlling system as shown in FIG. 1 comprises an internal combustion engine 10, an inlet manifold 11, an exhaust manifold 12, and an exhaust pipe 13. An exhaust gas sensor 14 is located at a portion of the exhaust pipe 13 for detecting the air fuel ratio. A three-way catalytic converter 15 is connected in the exhaust pipe 13 downstream of the exhaust gas sensor 14. A controlling circuit generally designated at 31 comprises a deviation detector circuit 16 having one input supplied with the output of the exhaust gas sensor 14 and another input supplied with the output of a reference value generator circuit 17 generating a reference value of a fixed level to produce a deviation signal in accordance with the magnitude of the output of exhaust gas sensor 14, that is, the magnitude of air fuel ratio, and an air fuel ratio correcting circuit 18 receiving the deviation output of the deviation detector circuit 16 to produce an air fuel ratio correcting signal containing an integrated component output and a proportional component output of the deviation signal. An actuator 19 receives the air fuel ratio correcting signal of the engine to control in accordance therewith amount of fuel or auxiliary air to be supplied into the inlet manifold 11. FIG. 2 shows another well-known feedback type air fuel ratio controlling system, as disclosed in Japanese Patent Application Laid Open No. 133412/'77, for example, in which the air fuel ratio of a mixture in the engine inlet system is previously adjusted to a slightly richer value than a set value and supply of secondary air to the engine inlet system is feedback controlled by using the output of an exhaust gas sensor. This controlling system comprises as shown in FIG. 2 an internal combustion engine 20, an inlet manifold 21, and an exhaust manifold 22 mounted with a secondary air distributor pipe 23 through which secondary air is supplied into the exhaust manifold. Numerals 13, 14 and 15 respectively designate an exhaust pipe, an exhaust gas sensor, and a three-way catalytic converter as in FIG. 1. A controlling circuit generally designated at 32 comprises a deviation detector circuit 16 similar to that of FIG. 1 while dispensed with the air fuel ratio correcting circuit 18 of FIG. 1. Connected between an air pump 28 driven by the engine and the secondary air distributor pipe 23 is a secondary air actuator 29 which receives the output of the controlling circuit 32 and controls in accordance therewith supply amount of the secondary air. It is appreciated that the secondary air actuator 29 may be also designed to perform the function of the air fuel correcting circuit 18 of FIG. 1. Turning to FIG. 3 showing an output characteristic of the exhaust gas seansor 14, it will be understood that the exhaust gas sensor 14 produces an output which is about 0.9 volts when the total air fuel ratio is richer than the stoichiometric air fuel ratio (corresponding to an excess air ratio .lambda.=1) and is about 0.1 volts when the total air fuel ratio is leaner than the stoichiometric air fuel ratio (.lambda.=1), and which varies abruptly around .lambda.=1. Accordingly, by comparing the output of the exhaust gas sensor 14 with a reference value V.sub.R of about 0.5 volts, it is possible to detect whether the total air fuel ratio is richer or leaner than the stoichiometric air fuel ratio representative of a target air fuel ratio of the three-way catalytic converter 15. Thus, since the actuator 19 of FIG. 1 is operated through the air fuel ratio controlling circuit 18 or the secondary air actuator 29 of FIG. 2 is operated in accordance with a result of the aforementioned comparison, supply amount of air is increased or supply amount of fuel is decreased to cause the total air fuel ratio to turn to a lean value when the detected total air fuel ratio is richer than the target value whereas supply of air is decreased or supply of fuel is increased to cause the total air fuel ratio to turn to a rich value where the detected total air fuel ratio is leaner than the target value, thereby making it possible to maintain the total air fuel ratio within a narrow range around the target value. In this process, it takes some time for the exhaust gas sensor 14 to detect a resultant total air fuel ratio corrected by the actuator 19 or the secondary air actuator 29 because a time (hereinafter referred to as a transportation delay) is required of the mixture or the exhaust gas for its movement from a location at which the actuator 19 or the secondary air actuator 29 is placed to a location at which the exhaust gas sensor 14 is placed and because the exhaust gas sensor 14 has an inherent detection delay, resulting in a time delay responsible for a cyclic variation in the total air fuel ratio within a width about the center of the target air fuel ratio. This variation is illustrated in FIG. 4 where the abscissa represents time, solid curve A an output of the exhaust gas sensor 14, solid curve B a total air fuel ratio at the location at which the actuator 19 or the secondary air actuator 29 is placed, V.sub.R a reference value, .lambda.=1 a level representative of a target value, that is, the stoichiometric air fuel ratio, T.sub.AL a time delay required of the exhaust gas sensor 14 starting from a time when the total air fuel ratio turned lean at the location at which the actuator 19 or the secondary air actuator 29 is placed to a time when the exhaust gas sensor 14 detects it, that is, a time delay required of the output of exhaust gas sensor 14 for its decrease below V.sub.R, and T.sub.AR a time delay required of the exhaust gas sensor 14 starting from a time when the total air fuel ratio turned rich at the location at which the actuator 19 or the secondary air actuator 29 is placed to a time when the exhaust gas sensor 14 detects it, that is, a time delay required of the output of exhaust gas sensor 14 for its increase beyond V.sub.R. The gradient of solid curve B corresponds to an air fuel ratio changing rate (a controlling gain) which is determined by a characteristic of the air fuel ratio correcting circuit 18 or that of the secondary air actuator 29. Further, an air fuel ratio variation width .DELTA..lambda..sub.A is determined by both the air fuel ratio changing rate and the time delays T.sub.AL and T.sub.AR. As seen from FIG. 4, as the air fuel ratio changing rate and time delays T.sub.AL and T.sub.AR become small, the air fuel ratio variation width becomes small correspondingly, thereby ensuring that the exhaust gas purifier can be operated effectively.
Incidentally, it is known in the art that the time delays T.sub.AL and T.sub.AB can be decreased to T.sub.BL and T.sub.BR, respectively, by setting a higher level V.sub.1 than V.sub.R, at which level V.sub.1 the exhaust gas sensor 14 detects a change of rich air fuel ratio to a lean air fuel ratio, and a lower level V.sub.2 than V.sub.R, at which level V.sub.2 the exhaust gas sensor 14 detects a change of a lean air fuel ratio to a rich air fuel ratio. In this expedient, the output of exhaust gas sensor 14 varies as shown at dotted curve C in FIG. 4 and the total air fuel ratio varies as shown at dotted curve D in FIG. 4. Thus, the air fuel ratio variation width .DELTA..lambda..sub.A is decreased to .DELTA..lambda..sub.B, ensuring that the exhaust gas purifier cam be operated with higher efficiency.
A feedback type air fuel ratio controlling system having the ability of providing the reference value, with which the output of exhaust gas sensor is compared, with a hysteresis characteristic is known, as disclosed in Japanese patent application Laid Open No. 114821/'77, for example. In this prior art controlling system, three types of conditions are judged including the exhaust gas sensor output being either above or below V.sub.R, the exhaust gas sensor output being either in increase or in decrease, and the exhaust gas sensor output being either within .+-..DELTA.V range about the center of V.sub.R or not (V.sub.R +.DELTA.V corresponding to V.sub.1 and V.sub.R -.DELTA.V corresponding to V.sub.2), and these conditions are processed by logical circuits to obtain the aforementioned hysteresis characteristic. With this prior art controlling system, however the number of detecting conditions are so large that complicated logical circuits are required. Further, in the event that the exhaust gas sensor is aged or subjected to varying ambient temperatures and its output assumes a waveform as shown at chained curve E in FIG. 4, failure to obtain the hysteresis characteristic results with such a problem that only a differentiation value of the exhaust gas sensor output permits the feedback controlling to be performed.